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propensity

Tendency [to a particular kind of behavior.
A person's propensity to joke is their natural tendency to joke, that is, they have the behavior of joking. A person has a propensity to lie if they have the natural tendency to lie with or without good reasons. If a person makes much of very little things, they have the propensity to exaggerate.

The teacher noticed in the boy a propensity to bully others. Not a day passed without his bullying his class mates especially the milder pupils of the class.

His propensity to jokes was so strong that even in the most serious situation he would joke and that irritated others.